The proposed research involves determination of endogenous opioid peptide effects on cerebral systems regulating blood pressure and hydration in the rat. Both pharmacologic and regulatory stimuli will be used to activate hydration responses. An acute elevation in CSF (NaCl) or angiotensin II stimulated increase in drinking behavior, blood pressure, and plasma vasopressin concentration are inhibited (dose and time related; naloxone-sensitive) by beta-endorphin or enkephalin administered into the left lateral cerebroventricle (LLV). Studies are designed to determine the site (cerebroventricular sensitivity; microinjection into specific brain regions) and possible mechanism (Na, K ion ATPase activity, inhibition neurotransmitter release) by which opioid peptides influence CNS systems regulating blood pressure and hydration. "Specific" opiate receptor antagonists, naloxone and naltrexone, will be used to determine when endogenously synthesized opiate receptor ligands influence vasopressin neurosecretion, in vivo. In addition, beta-endorphin effects on drinking behavior produced by regulatory stimuli (osmotic, hypovolemia, food-associated dehydration) will be determined. Proposed studies also include determination of opioid peptide effects on blood pressure and hydration in spontaneously hypertensive rats (SHR).